As announced earlier that the MIUI 8 beta v 7.5.18 is coming to RN3, So here is it... if you are on Beta ROM you will get update via OTA i.e. Settings>About phones>System Updates and in case the update has not reached you or you are impatient then here are the links......to download the ROM

For flashing the Fastboot ROM you need to download the MiFlash tool. You can download the flash tool from Here

Also you need to unlock your device if you are flashing the Fastboot ROM

Please go to "Updater" and choose full package zip file to upgrade. If you are updating from other ROMs, please don't forget to back up and wipe all data before flashing to avoid data conflicts. Use Clean All option in the new Mi Flash Beta Tool while flashing to perform a full wipe and fresh install.